1. Field of the Invention
The present invention relates to a method for making polythiourethane substrates and in particular optical substrates such as ophthalmic lenses.
More specifically, the present invention relates to a method for making polythiourethane based optical substrates such as ophthalmic lenses having a high refractive index nD25 of 1.60, preferably of 1.65 and more preferably of 1.67 or more which are substantially free from optical defects such as bubbles and/or striations resulting from the polymerization process.
2. Description of the Prior Art
Polythiourethane substrates such as ophthalmic lenses are typically made by filling a moulding cavity with a liquid mixture of appropriate monomers, such as a mixture of a polyisocyanate and a polythiol, polymerizing the monomer mixture and thereafter recovering the polymerized polythiourethane substrates from the mould.
One major problem associated with these prior art moulding methods, especially for making optical substrates such as ophthalmic lenses, is the obtention of a defect free substrate, in particular free from striations.
If the liquid monomer mixture is of relatively low viscosity, i.e. less than 3.10−3 Pa.s at 25° C., time to gelation is relatively long. In that case, local heat points appearing within the mixture during polymerization create convection currents resulting in the so-called “convection induced striations” within the final polymerized substrate.
The rate of formation of such convection induced striations increases when the mixture viscosity decreases.
On the other hand, the mere filling of the mould cavity with the monomer mixture creates the so-called “filling induced striations”. The relaxation time necessary to eliminate such filling induced striations increases with the increase of the monomer mixture viscosity.
Furthermore, when using monomer mixtures of relatively high viscosity, air bubbles which may be entrapped within the liquid monomer mixtures are difficult to eliminate.
Thus, the aim of the present invention is to provide a method of fast curing a polythiourethane transparent casted substrate which remedies to the drawbacks of the prior art methods.
Another object of the invention is to provide a method of fast curing polythiourethane transparent casted substrates substantially free from optical defects, in particular free from convection and/or filling induced striations.